Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is an enlarged front view of a first image in a sequence according to a first embodiment of my design for a display screen or portion thereof with a transitional graphical user interface; and

FIG. 2 is an enlarged front view of a second image in the sequence thereof.

FIG. 3 is a front view of a first image in a sequence according to a second embodiment of my design for a display screen or portion thereof with a transitional graphical user interface; and,

FIG. 4 is a front view of a second image in the sequence thereof.

The appearance of the transitional image transitions between the images shown in FIGS. 1-2 and 3-4. The process or period in which one image transitions to another forms no part of the claimed design.

The dash-dot-dash broken lines show portions of the graphical user interface and form no part of the claimed design. The dash-dash broken lines show a display screen or portion thereof and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 